The present invention relates to a method of feedback-controlling the air-fuel ratio of an internal combustion engine, and more particularly, it relates to a method of this kind wherein the air-fuel mixture supplied to the engine is feedback-controlled to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics in approximate proportion to the exhaust gas ingredient concentration.
Conventionally, it is known to control the air-fuel ratio of a mixture supplied to an internal combustion engine to a value leaner than a stoichiometric ratio when the engine is in a predetermined operating condition. Further, an apparatus for controlling the air-fuel ratio for an internal combustion engine has been proposed by U.S. Pat. No. 4,732,130, in which the air-fuel ratio is controlled so as to become increasingly leaner than the stoichiometric air-fuel ratio as the speed reduction ratio decreases, in order to minimize the rate of fuel consumption while preventing the driveability from being deteriorated due to surges of the engine output occurring when the speed reduction ratio is large. In the meanwhile, among conventional methods for feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (hereinafter referred to as "supply air-fuel ratio") to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics proportional to the exhaust gas ingredient concentration, there is a method proposed by Japanese Provisional Patent Publication (Kokai) No. 63-12846 wherein in the case of setting the desired air-furl ratio to a value leaner than the stoichiometric air-fuel ratio, it is set to a leaner value as the reduction ratio of the transmission of the vehicle on which the engine is installed is smaller, in order to properly improve the rate of fuel consumption without impairing driving feeling or driveability.
However, the proposed apparatus and method suffer from a problem that when the travelling speed of the vehicle is low, the output torque of the engine becomes insufficient, which degrades the driveability of the vehicle.